It is often desirable to place a partial coating of protective material on workpieces such as assembled printed circuit ("PC") boards. Such coatings generally provide several functions, such as preventing short circuits caused by stray conductive debris and providing insulation against extreme temperatures and vibration.
One objective when applying such coatings is to avoid the introduction of the coating material into electronic components which may be adversely affected by the coating. Such components include, for example, potentiometers, variable capacitors, and other components with moving parts, as well as heat sinks and the like whose heat dissipation characteristics are hindered by coating material. Thus, it is generally desirable to coat the bottom of a PC board where soldering connections are made, and to also apply a thin or "shallow" coating to the top of the board where the components are attached to the board. However, the coating should not be allowed to "wick up" the components. In addition, coating should be excluded from any component, wherever located, which may be adversely affected by the coating.
Many techniques are known to accomplish such an application of a coating, but none are completely satisfactory. For example, coating may be sprayed on a PC board, but it is difficult, even with stencils and shields covering the board, to prevent the coating from getting on components which should not receive coating. In addition, spraying does not allow a thin layer of coating material to be placed on the upper surface of a board without also entirely coating the components. Another method is to partially immerse the board in a bath of coating material by placing it on supports in the bath. The supports allow the board to be submerged to a depth sufficient to coat the bottom and a slight amount of the top of the board. However, air bubbles can prevent the complete coating of bottom portions of a board, the coating tends to "wick up" the components on the board, the boards are subject to being accidentally immersed in the coating material when the board is being inserted or removed from the bath of coating material, and this submersion process is generally time-consuming. Other coating methods and apparatuses must be designed for a workpiece of a particular shape, but these may not allow convenient substitution of differently sized workpieces.